From WO 01/89758 A1 it is known to heat the sleeve section inductively through an annular coil assembly, substantially enclosing the sleeve section in a coaxial manner. The coil assembly is connected to a high frequency AC generator and induces Eddy currents in the metal sleeve section, heating said sleeve section. At the face sides, and on the circumference of the coil assembly concentrator elements, made from magnetically soft, substantially electrically non conductive material like ferrite or similar are disposed, which direct the magnetic flux generated by the coil assembly onto the sleeve section of the tool holder, and in particular into the portion of the free end of the sleeve section. The portions adjacent to the free end of the sleeve section of the concentrator arrangement are formed by radially moveable elements. The pass through enclosed by said elements can be adapted to the diameter of the tool holder in this manner. Said elements are supported on the axial face area of the free face end of the sleeve section during operation and thus determine the axial position of the coil assembly and of the concentrator assembly, otherwise connected with it into a unit, relative to the free end of the sleeve section during heating operation. This way the induction coil unit can be positioned axially relative to the tool holder.
From DE 101 57 432 A1 a similar induction coil unit is known, in which however the annular coil assembly enclosing the sleeve section during heating operation includes two coil sections, disposed at a fixed axial distance from each other. The axial portion between the two coil portions has no coils. This way it is accomplished that the sleeve section heats up less in its middle section, than in its end section.
The sleeve section of the tool holder has an axial length adapted to its use and the diameter of the sleeve section is selected according to the diameter of the tool shaft. In conventional induction coil units only the diameter of the pass through opening for the tool shaft, which is determined by the magnetic flux concentrator assembly can be adapted to the tool holder to be heated. Since the coil assembly has a predetermined constant axial length, either the coil assembly has to be replaced, or it can not be avoided that a portion of the tool holder which is not used for clamping the shaft of the tool, is also being heated.